The present disclosure relates generally to antennas for portable, handheld communication devices, and more particularly to assemblies of multiple antennas for such devices.
Different types of wireless mobile communication devices, such as personal digital assistants, cellular telephones, and wireless two-way email communication equipment, cellular smart-phones, wirelessly enabled notebook computers, are available. Many of these devices are intended to be easily carried on the person of a user, often compact enough to fit in a shirt or coat pocket.
As the use of wireless communication equipment continues to increase dramatically, a need exists for increased system capacity. One technique for improving the capacity is to provide uncorrelated propagation paths using Multiple Input, Multiple Output (MIMO) systems. A MIMO system employs a number of separate independent signal paths, for example by means of several transmitting and receiving antennas.
MIMO systems, employing multiple antennas at both the transmitter and receiver offer increased capacity and enhanced performance for communication systems without the need for increased transmission power or bandwidth. The limited space in the enclosure of the mobile communication device, however presents several challenges when designing such multiple antennas assemblies. An antenna should be compact to occupy minimal space and its location is critical to minimize performance degradation due to electromagnetic interference. Bandwidth is another consideration that the antenna designers face in multiple antenna systems.
Furthermore, since the multiple antennas are located close to each other, strong mutual coupling occurs between their elements, which distorts the radiation patterns of each antenna and degrades system performance, often causing an antenna element to radiate an unwanted signal. Thus, minimal coupling between antennas in MIMO antenna arrays is preferred to increase system efficiency and battery life, and improve received signal quality.
Previously electromagnetic band gap (EBG) structures have been used for various isolation purposes. EBG structures are periodic arrays of objects or cells that prevent the propagation of the electromagnetic waves in a specified band of frequencies. These arrays were either linear, in which a plurality of EBG cells were spaced at equal distances along a line, or two-dimensional, in which a plurality of EBG cells were spaced at equal distances in both of two orthogonal dimensions. In a common configuration where two antennas are located on a surface of a printed circuit board, a line of periodically spaced EBG cells extend between those antennas from one edge of the printed circuit board to and opposite edge. This forms a continuous signal barrier between the antennas. Both linear and two-dimensional arrays of EBG cells used a shotgun approach by forming a barrier along a relatively large section so that wherever a signal from an antenna may travel through that section one of more cells would block the path of the signal. Such an unfocussed shotgun approach affected the ground plane by the placement of significantly more EBG cells than were actually required for isolating signal emitted by an antenna from reaching another device.
In addition MIMO antenna arrays often are capable of being tuned to operate at several different radio frequency bands and their operating frequency and parameters can be changed based on system requirements. Conventional EBG cells on the other hand, are designed to operate at a single stop band of frequencies. Therefore such EBG cells provided less than optimal isolation when the antennas are tuned and operating at a different frequency band than the stop band of EBG cells.
Therefore, it is desirable to develop a MIMO antenna arrangement which has a compact size to fit within a communication device housing and has a high level of isolation between the antennas for optimal performance across all frequency bands of operation.